The present invention relates to a transition from microstrip to waveguide. A particular, but not exclusive, application for such a transition is to enable energy in RF circuits fabricated in microstrip to be coupled to an antenna port generally formed by waveguide.
U.S. Pat. No. 4,754,239 discloses a waveguide to stripline transition. A stripline comprises two strip conductors, one on each side of a dielectric sheet. The transition described and shown comprises a length of stripline secured to the underside of the floor of an overlapping end of a length of rectangular waveguide. Within the waveguide and mounted on the ceiling, symmetrically of the longitudinal plane of symmetry, is a substantially right angled triangular ridge having a terminal end portion connected to one of the stripline conductors. The transition is tuned to provide an optimum impedance and voltage standing wave ratio (VSWR) by moving a reflecting panel behind the wedge along the waveguide from an open end.
WO98/11621 discloses a transition from a waveguide to a strip transmission line which is fabricated as a one-piece component. A web located within and symmetrically of the longitudinal plane of symmetry of the waveguide reduces the cross section of the waveguide in a direction towards the strip transmission line to which it is connected electrically. The cross-section of the web also tapers symmetrically towards the connection to the strip transmission line. The longitudinal profile of the web may take several forms including stepped, straight, convex, concave and double taper.
In both these prior transitions, there is a risk of premature hard metal to metal contact which will tend inhibit reliable contact between the tapered ridge or web and the stripline conductor.
An object of the present invention is to improve the electrical contact in waveguide to microstrip transitions.
According to a first aspect of the present invention there is provided a transition from microstrip to waveguide, comprising a rectangular waveguide composed of a floor separated from, but electrically connected to, side walls and a ceiling, a microstrip structure including a resilient substrate coextensive with the floor and separating the side walls from the floor, and an electrical contact depending from the ceiling and contacting the microstrip structure.
The first aspect of the present invention provides a transition from microstrip to waveguide, comprising a rectangular waveguide comprising a ceiling integrally formed with side walls and a separate floor and a microstrip structure formed by a resilient substrate having a stripline conductor on one surface and a ground plane conductor on an opposite surface, wherein the substrate is disposed between the sidewalls and the floor with the planar conductor contacting the floor, the stripline conductor extends along the longitudinal plane of symmetry for a portion of the length of the waveguide from one end thereof, means are provided for electrically connecting the floor to the sidewalls, and a tapered ridge depends from the ceiling and extends along the longitudinal plane of symmetry of the ceiling in a direction from the other end thereof, a terminal end of the ridge contacting a terminal portion of the stripline conductor.
According to a second aspect of the present invention there is provided a combination of a microstrip RF circuit, a waveguide for connection to an antenna and a transition from microstrip to waveguide, the transition comprising a transition made in accordance with the first aspect of the present invention.
As the microstrip structure includes a resilient substrate, the side walls bear onto a relatively soft surface which prevents premature contact between hard metal surfaces which would be the case if the substrate was dimensioned to fit within the area bounded by the side walls. As a result a good electrical connection between the depending electrical contact and the microstrip structure can be effected under mechanical pressure.